1. Field of the Invention
This invention relates to recyclable and repulpable coated paper stocks and processes for manufacture of such coated paper stocks having high moisture vapor barrier characteristics. This invention relates to coated paper stocks having low moisture vapor transmission rates. In particular the invention relates to a coated paper stock having moisture vapor resistance and fashioned from components compatible with recycling and repulping. The moisture vapor resistant paper stock is comprised of a substrate coated on at least one surface with a subcoat and a top coat both of which are applied as water based dispersions. The water-based dispersion coating components of the invention yield a coated paper stock that is recyclable and repulpable more readily than conventional single layer extruded or laminated polyethylene coatings, and exhibits surprising vapor barrier properties.
2. Description of the Related Art
Polyethylene films extruded or laminated to paper and wax coatings coated to paper are extensively used in packaging applications to protect dry grades from moisture and provide water and grease resistance. Such coating however are generally difficult to repulp and recycle. It is desirable to find an alternate to such films and coatings that have at least some comparable barrier properties but having the added benefit of being easier to recycle and repulp.
U.S. Pat. No. 3,196,038 to Schoch et al. discloses continuous application of multiple coatings (at least three) on a paper substrate to render the paper web moisture and grease resistant. The base coating is a dispersion selected from the group consisting of polyvinyl chloride, polyvinyl acetate, polyvinylidene chloride, esters of polyacrylic acid, polystyrol, polyethylene, paraffin and mixtures thereof. The second coat, preferably of the same composition as the base coating, provides a flexible intermediate layer to which a harder top coat is then applied.
U.S. Pat. No. 3,873,345 to Vreeland discloses a process for producing a high gloss paper with a heat calendering apparatus. The coating composition taught is comprised of a mineral pigment, such as clay and a binder selected from the group consisting of vinyl acetate, styrene-isoprene and acrylic polymer latices.
U.S. Pat. No. 4,248,939 to Parsons teaches treating paper for purposes of improved printability with a first layer comprised of an inorganic pigment, such as clay, and binder, and a second layer comprised of an organic pigment and binder. The organic pigments used in the second top coat are prepared by graft copolymerizing in aqueous medium a monoethylenically unsaturated monomer onto a water soluble prepolymer. The water soluble prepolymers used include copolymers of vinyl alcohol and acrylic acid. The organic pigment coating compositions are used to create high gloss paper under less severe calendering conditions than coating systems using inorganic pigments alone.
U.S. Pat. No. 4,328,284 teaches spreading of a single layer coating composition consisting of a latex and major amounts of pigment. Only minor amounts of binder such as starch and protein are suggested for use in the composition, though no protein based composition is illustrated.
U.S. Pat. Nos. 4,265,969 and 4,301,210 to Yasuda et al. disclose processes for preparing cast coated paper having high gloss. The undercoating contains pigment binders and the aqueous overcoating consists of pigments with polymer latex binders having glass transition temperatures over 38° C.
U.S. Pat. No. 4,812,550 teaches a process for preparing grafted proteins wherein ethylenically unsaturated monomers are subjected to a free radical polymerization in an aqueous medium in the presence of solubilizing proteins. The latices thus obtainable are taught useful as binders for pigmented paper coating compositions.
U.S. Pat. No. 4,844,952 to Korenkiewicz et al. relates to multicoated substrates having stain and grease resistance. A conventional decorative finish coating composition, which is solvent based or aqueous, is applied to the substrate followed by application of a coating of a waterborne polymer composition free of external crosslinking agents.
U.S. Pat. No. 4,997,862 teaches a method of cast coating a paper described as having high gloss, flexibility, smoothness, brightness, and resistance to drum adhesion and pick resistance comprising preparing an aqueous paper coating composition of paper coating pigments and a soy protein synthetic graft copolymer adhesive binder.
U.S. Pat. No. 5,169,715 to Maubert et al. discloses a high gloss paper produced by applying two coats. The first base coat comprises a synthetic latex, starch, polyvinyl alcohol and proteins. The second gloss coating contains conventional pigments such as calcium carbonates, kaolins, talcs, calcium sulphates, silicoaluminates, satin whites, silicas, aluminas and aluminum hydroxides and binders. Acrylic styrene copolymers are used as gloss pigments.
U.S. Pat. No. 5,260,396 teaches a process for preparing a water-resistant film or coating from a water soluble or dispersible grafted protein comprising polymerizing by a free radical mechanism a mixture consisting essentially of a monoethylenically unsaturated monomer and a protein to form a grafted protein, and forming a film or coating from the grafted protein. The film is then treated or coated with di- or tri-valent salts, glyoxal or glutaraldehyde.
U.S. Pat. No. 5,435,841 teaches compositions for insolubilizing the binders in coatings for paper. This patent describes paper coating compositions as generally a fluid suspension of pigment, such as clay with or without titanium dioxide, calcium carbonate, or the like, in an aqueous medium which includes a binder, such as starch, protein, or latex, to adhere the pigment to the paper. A process for preparing an insolubilizer involving a glyoxal-urea adduct is described.
U.S. Pat. No. 5,654,039 to Wenzel teaches a recyclable and compostable paper stock comprising a primer coat of a water dispersion of a polymer selected from acrylic polymers, acrylic copolymers, polyvinyl acetate, polyvinyl alcohol, ethylene-vinyl acetate, polyethylene vinyl chloride, styrene butadiene copolymers, polyvinylidene chloride and starch. A top coat of a wax and a water dispersion of a polymer selected from acrylic polymers, acrylic copolymers, styrene-butadiene copolymers and polyvinylidene chloride is taught as forming a coated paper stock resistant to grease and having moisture barrier characteristics.
The same assignee later in U.S. Pat. Nos. 6,548,120 and 5,989,724 then asserts that the very same polymers in a base coat and top coat, this time wax free, are also recognized to have moisture barrier characterics.
Despite the above teachings, no water emulsion based moisture barrier coated papers are believed to be widely used commercially. Existing recyclable moisture barrier coated products have gained only limited acceptability.